You do not need reservations for this train. Just a ticket. Unless you have a pass in which case you do not need anything else.
These are normal service trains, used by people to go about their daily busines.
Furthermore there is no way yo have a forward facing seat as the train reverses halfway. So every seat faces one way half of the trip, and then the other way. When you board in Luzern just pick a free seat. Best views are on the right.